1. Field of the Invention
This invention relates generally to abatement of undesirable components such as fluorine, silane, gaseous fluorides, acid gases, hydride gases and halide gases from effluent streams containing same, and more specifically to the use of systems employing a wet scrubber apparatus and method for abating undesirable components of the aforementioned type in semiconductor manufacturing processes.
2. Description of the Related Art
In point of use wet scrubbing abatement of semiconductor off-gases, various applications require the removal of hydride gas, acid gas, and entrained solids. This is especially true in processes that use or produce SiH4 (silane), NH3 (ammonia), F2 (fluorine), HF (hydrogen fluoride), SiF4 (silicon tetrafluoride), or COF2 (carbonyl fluoride), such as certain CVD (chemical vapor deposition) processes.
In these effluent gas stream treatment applications, the art has typically employed a multiple component scrubbing system. In such a device, the silane and optionally the ammonia are thermally oxidized in one module of the abatement system, and the HF, F2, SiF4, COF2, and optionally NH3 are scrubbed using water in another, separate module. Disadvantages of thermal oxidation include (i) high energy consumption, and (ii) the generation of NOx resulting from the oxidation of ammonia. In addition, high temperature heated modules may accelerate corrosion downstream of the thermal module because the acid gases (F2 and HF) are heated, but not abated in the thermal unit. Typically, a water scrubbing module is located directly downstream from the thermal module. It is in the hot, moist interface region between the water scrubbing unit and the thermal unit that the hot acid gases typically cause corrosion.
There is therefore a compelling need for a simple, reliable abatement device that can effectively treat effluent streams containing gas species of the type mentioned above.
More specifically, concerning fluorocompounds as effluent gas species that are desirably abated in treatment of effluent gas streams containing same, perfluorinated gases are widely used in chip manufacturing to generate in-situ F2 and fluorine radicals using plasma-assisted reactions. These highly reactive species are produced to remove silica from tool chambers or to etch materials such as nitrides, oxides, or polysilicon from wafers. The most commonly used carbon-based perfluorinated species include CF4, C2F6, and C3F8. Nitrogen trifluoride (NF3) and sulfur hexafluoride (SF6) are also widely used.
Perfluorinated compounds (PFCs) are also among the strongest greenhouse gases with global warming potentials (GWPs) three and four orders of magnitude higher than CO2. Moreover, PFCs are extremely stable molecules having lifetimes in the atmosphere of thousands of years. Even though the semiconductor industry is not the largest source of PFC emissions, the industry is actively pursuing strategies to reduce PFC emissions and to protect the environment.
Ongoing research to reduce PFC emission levels falls into four categories: optimization, use of alternative chemicals, recovery/recycle techniques, and abatement processes.
Process optimization involves adjusting the operating conditions in the reactor to achieve enhanced PFC conversion within the semiconductor manufacturing tool. Existing non-optimized conditions in the semiconductor manufacturing process result in PFC utilization that varies depending on the specific gas and process used. For instance, oxide etches using a combination of CF4 and CHF3 rank lowest with 15% process efficiency. Tungsten deposition processes are reported to utilize up to 68% of NF3. Recent developments in optimized plasma clean technologies were demonstrated to provide up to 99% NF3 utilization within the semiconductor manufacturing tool.
High PFC conversions inevitably result in the formation of hazardous air pollutants (HAPs). Breakdown products include mostly fluorine (F2) and silicon tetrafluoride (SiF4) gases and to a lesser extent HF and COF2. Destruction of fully fluorinated gases generates considerably augmented HAP yields compared to the initial PFC volumes delivered to the semiconductor manufacturing tool. For instance, assuming stoichiometric conversion of PFCs into F2, a 1 liter per minute (lpm) flow rate of NF3 could potentially produce 1.5 liters per minute (lpm) of F2. The combined exhaust stream of four chambers in a semiconductor manufacturing process system could potentially generate up to 6 standard liters per minute (slm) of fluorine gas resulting in a post-pump effluent concentration of 3% F2 (50 lpm ballast N2 per pump).
These estimated values double with hexafluorinated PFCs (compared to NF3) and are likely to increase in the future with the projected throughputs of 300 mm wafer manufacturing. These estimates represent worse case scenarios and do not account for the short duration and periodic nature of processes using PFCs, the lower concentrations of F2 emissions during initial cleaning stages, and the reduced probability that two or more chambers run PFC cycles synchronized. Nonetheless, such estimates indicate the serious and worsening character of the PFC problem associated with semiconductor manufacturing operations.
The toxic and corrosive nature of fluorinated HAPs pose considerable health and environmental hazards in addition to jeopardizing the integrity of exhaust systems. In particular, the oxidizing power of F2 is unmatched by any other compound used or generated in the semiconductor manufacturing facility, and is far more reactive than other halogens. The large volumes of F2 and other fluorinated hazardous inorganic gases released during optimized plasma processing require the utilization of point of use (POU) abatement technologies in order to minimize potential dangers and to prolong tool operating life.
There are several potential alternative methods for point of use F2 abatement. At high concentrations, fluorine reacts exothermically with all elements except O2, N2, and noble gases. Consequently, a reasonable approach to F2 abatement is to remove this highly active gas using naturally-occurring reactions without adding energy to the system. The main challenges to this potential approach are heat dissipation and forming acceptable by-products.
Alternative fluorine abatement techniques affording potential solutions to the fluorine abatement problem include wet as well as dry reaction techniques, and thermal reaction techniques.
In dry processing, the fluorine gas stream is flowed through a dry bed filled with a reactive material. Suitable dry chemicals would convert F2 into innocuous solids or benign gases without generating excessive heat. This last condition could be a limiting factor especially when large volumes of F2 are involved.
In a thermal reaction approach, thermal abatement units combine reactive materials and F2 inside a reactor heated using fuel or electrical energy. The by-products generated by the thermal abatement of F2 typically include hot acids requiring the use of a post-reaction water scrubber. The removal efficiencies in these post-reaction water scrubber beds are often compromised, inasmuch as the scrubbing efficiency of most acid gases decrease as a function of temperature. In addition, containment of hot concentrated acids requires expensive materials and construction to prevent temperature-enhanced corrosion attack.
In wet processing techniques, advantage is taken of the fact that fluorine gas reacts quickly and efficiently with H2O. The main products of the reaction between water and F2 are HF, O2, and H2O2. Objections to using water scrubbers include concerns over the formation of unwanted OF2, and the water consumption necessary to achieve acceptable removal efficiencies at high fluorine challenges.
Comparison of the foregoing treatment options shows that wet scrubbing techniques are potentially the most attractive, provided that the OF2 by-product formation and high potential water consumption problems can be resolved.
There is, accordingly, a need in the art for a point of use wet scrubber fluorine abatement system that inhibits the formation of unwanted OF2, that has an acceptable fluorine removal efficiency at high fluorine concentrations and that concurrently minimizes water usage.
Considering now silanes as undesirable components of effluent gas streams that are desirably abated in gas stream treatment, such components as mentioned above are typically removed by thermal oxidation. Water scrubbing removal of silanes has generally not been considered advantageous in comparison to thermal oxidation, because of the very low solubility of silanes in water and their very low reactivity with water. The prior art in some instances has used chemicals such as KOH and NaOH for such scrubbing, but scrubbing silanes with such hydrides generally requires large amounts of the chemical additives and therefore entails substantial operating costs. Chemical scrubbing is described for example in xe2x80x9cEfficiently handling effluent gases through chemical scrubbing,xe2x80x9d T. Herman and S. Soden, American Institute of Physics Conference Proceedings 166, Photovoltaic Safety, Denver, Colo. 1988.
In addition to the foregoing approaches for achieving abatement of silanes, there are commercially available certain devices that effect thermal oxidation of silane prior to final scrubbing of the effluent gas in a water scrubber. These devices, however, suffer from the disadvantage of requiring ignition sources and fuel, or alternatively electricity, for heating. The associated process also tends to be highly exothermic in nature, resulting in excessive temperatures and substantial exhaust gas quenching requirements.
Another problem experienced in abatement of silanes is that ammonia gas may also be present in the effluent gas stream. The concurrent presence of silane and ammonia presents particular difficulty in achieving high levels of abatement of these components.
There is therefore a need in the art for a gas abatement system that can effectively abate silane and ammonia gas when both are simultaneously present in the effluent gas stream.
Accordingly it would be a significant advance in the art to provide a means and method for efficient removal of silanes that avoid the disadvantages of thermal oxidation treatment.
It would also be a significant advance in the art to provide effect removal of silane at ambient or near-ambient temperature levels, or otherwise at temperature conditions that are substantially below those employed in thermal oxidation treatment. There is therefore a need for a xe2x80x9ccold combustionxe2x80x9d method and apparatus for effecting abatement of silanes by low temperature oxidation thereof.
A further problem that has plagued the use of water scrubbers for the treatment of effluent gas streams is foaming. In certain semiconductor applications, effluent gases can cause foam formation when entering a water scrubber and such foam can cause deleterious effects inside the scrubber. The most serious problem occurs when the foam builds up in such a large quantity as to completely fill the interior volume of the scrubber. When this happens, foam becomes entrained in the gaseous phase and actually can be carried out of the scrubber. Where the foam coalesces on the exhaust pipe surfaces, corrosion can occur. Additionally, foaming can cause cavitation when foam is present in the sump liquor of the scrubber, and the foam thereby can damage the pump that recirculates the scrubbing liquor. Finally, such foaming activity may significantly increase the pressure drop across the scrubber and thereby adversely affect the operation not only of the scrubber and effluent treatment system, but also the operation of upstream semiconductor manufacturing units that are pressure-sensitive in character.
Yet another problem encountered in the operation of water scrubbers for effluent gas treatment is the mineralic content of the water used in the scrubber. In certain locations of the world and the United States, the make-up water supplied to water scrubbers is very hard, i.e., it contains a high concentration of calcium and magnesium and other ionic species. It has been found that when the water scrubber is operating with a pH above about 8.5, the calcium in the water tends to precipitate out as calcium carbonate (CaCO3). This creates a number of problems. One is that CaCO3 adheres to sensitive surfaces within the recirculation pump associated with the scrubber. This can cause the pump to seize up and fail. Another problem is that the CaCO3 deposits build up on the packing surfaces of the scrubber. This in turn causes an increase in pressure drop across the scrubber and a decrease in scrubbing efficiency. Finally, CaCO3 deposits may form in the water lines of the scrubber, causing an increase in pressure drop and therefore a reduction in water flow rate.
Another solids deposition problem of a more general character is the clogging by solids of lines connected to pressure sensing devices in the abatement system. Such lines are used to measure the pressure at the inlet of the abatement unit, to give the facility engineers an indication of whether any clogging is present in the abatement system. The lines (pressure sensing ports) can sometimes become clogged by particulates or by condensable gases in the effluent stream. If solids build up in the sensing line, the reading of the associated pressure sensing device will be inaccurate and may give a false alarm signal causing the abatement system to be shut down.
A related problem is the occurrence of solids deposition in the entry to the water scrubber, which may be attributable to the presence of condensable gases in the effluent gas stream being processed.
It therefore is desirable to minimize or eliminate the incidence of solids formation in the abatement system, to avoid or at least ameliorate the foregoing solids deposition problems.
In the point of use wet scrubbing abatement of semiconductor off-gases, where both acid gas removal and solids removal are required, e.g., in processes that use or produce Cl2, F2, HF, HCl, or NH3, such as metal etch, LPCVD, EPI, and CVD processes, the scrubbing system utilizes a single packed column through which the gas is flowed for treatment. Above the packed column is a spraying mechanism used to wet the packing material with the scrubbing liquor (usually water). The gas may pass downward through the column in the same direction as the falling water (cocurrent), or upward against the falling water (countercurrent). There is an advantage to using a countercurrent design because the water at the gas outlet (column top) is clean and enables maximum scrubbing potential. On the other hand, the water at the gas outlet of a column operated in cocurrent fashion (column bottom) can be saturated with the given acid gas, thereby limiting the scrubbing potential.
Unfortunately, the column size, packing wetting requirements, and effective solids removal demand that a significant water flow rate must pass over the packing in either cocurrent or countercurrent operation. Typical water flow rates through the packing typically will be in excess of 10 gallons/minute. Using such a high flow rate of fresh water is undesirable in terms of cost and also due to the significant consumption of water by the process facility, particularly in regions where water is scarce. The common answer to this dilemma is to use a recirculation pump to recycle used water back to the top of the packed column. The fresh water (make-up) flow rate can then be lowered. However, recirculation decreases the scrubbing efficiency of the scrubber for the aforementioned gas species.
One method used to increase scrubbing efficiency and decrease the fresh make-up water flow rate is to use a chemical injection agent. These materials work by reacting with the solubilized gases, thereby allowing additional gas molecules to enter into the aqueous scrubbing liquor as a result of the maximized mass transfer gradient. However, the use of chemical agents in this approach is costly and can present additional safety concerns.
It would therefore be desirable to provide a scrubbing system for the effective removal of both solids and acid gases, which does not require the use of chemical addition agents. It would also be desirable to provide a scrubbing system for the treatment of effluent gas, which allows for a significant reduction in the required fresh make-up water flow rate as compared to a typical water scrubber that does not employ the use of chemical addition agents.
Accordingly, it is an object of the present invention to resolve the above-discussed problems associated with effluent gas treatment systems of the prior art.
It is another object of the invention to provide an effluent gas treatment system overcoming such problems of the prior art.
It is a further object of the present invention to provide an effluent gas treatment system that employs a water scrubber in a highly efficient manner.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.
The present invention relates to an apparatus and method for abatement of undesirable components from effluent streams containing same.
Such undesirable components may variously include fluorine, silanes, gaseous fluorides, perfluorocarbons, acid gases, hydride gases and halide gases. Specific examples of such gas components include, without limitation, SiH4 (silane), NH3 (ammonia), F2 (fluorine), HF (hydrogen fluoride), SiF4 (silicon tetrafluoride), and COF2 (carbonyl fluoride).
The invention relates more specifically to effluent gas treatment systems employing a wet scrubber apparatus and method for abating undesirable components of effluent gas streams deriving from semiconductor manufacturing operations.
In one aspect, the present invention relates to a scrubbing system for the abatement of a gas component in a gas stream containing same, such scrubbing system comprising a gas/liquid contacting chamber including means for introducing to the contacting chamber the gas stream and a scrubbing liquid for gas/liquid contacting therein, and additionally at least one of the features of:
(a) a chemical injector for introducing a chemical reagent for contact with the gas component to remove same from the gas stream in said gas/liquid contacting, optionally in combination with a back pressure inducing device, e.g., an orifice to prevent or at least partially reduce foaming in the scrubbing system incident to chemical reagent injection;
(b) an inlet arranged for introduction to the gas stream flowed therethrough of a gas to enhance removal of silane from the gas stream when present therein;
(c) a second gas/liquid contacting chamber receiving a treated gas stream from the first gas/liquid contacting chamber and including means for introducing to said second contacting chamber a second scrubbing liquid for gas/liquid contacting therein, wherein the first gas/liquid contacting chamber is constructed and arranged for cocurrent flow of the gas stream and scrubbing liquid and wherein the second gas/liquid contacting chamber is constructed and arranged for countercurrent flow of the gas stream and the second scrubbing liquid;
(d) an antifoam agent injector for introducing to scrubbing liquid for said gas/liquid contacting a foam-suppressing antifoam agent, to suppress foam production in the scrubbing chamber, optionally in combination with a back pressure inducing device, e.g., an orifice to prevent or at least partially reduce foaming in the scrubbing system incident to antifoam agent injection;
(e) means for suppressing deposition of calcium carbonate from scrubbing liquid containing calcium, said suppressing means being selected from the group consisting of:
(1) a magnetization zone for imposing a magnetic field on scrubbing liquid prior to use thereof in the contacting chamber;
(2) means for adjusting the pH of the scrubbing liquid to maintain pH thereof below 8.5;
(3) a lime-soda ash bed arranged for flow of the scrubbing liquid therethrough prior to use of the scrubbing liquid in the contacting chamber; and
(4) a precipitator for precipitating the calcium content of the scrubbing liquid prior to use of the scrubbing liquid in the contacting chamber;
(f) means for suppressing solids formation in a passage of the scrubbing system, selected from the group consisting of means for flowing a purge gas through the passage to suppress solids formation therein, and means for heating the passage to suppress solids formation therein; and
(g) means for abating silane from the gas stream when present therein in combination with ammonia, such means being selected from the group consisting of:
(1) means for heating the gas stream prior to introduction of same to the scrubbing system; and
(2) a second gas/liquid contacting chamber according to (c) hereof, and means for introducing clean dry air or other oxygen-containing gas to the treated gas stream from the first gas/liquid contacting chamber prior to introduction thereof to the second gas/liquid contacting chamber.
Another aspect relates to a scrubbing system including an inlet structure for introducing to a scrubbing apparatus a gas stream containing a silane component. In such aspect, the gas stream is flowed through the inlet structure, and the inlet structure includes means for introducing a gas to the gas stream to enhance removal of the silane component in the scrubbing system. The gas may comprise clean dry air (or any other suitable oxygen-containing gas). The gas may be introduced to the silane containing gas stream in any suitable manner, e.g. bubbling into tipover water in a water overflow inlet structure, or bubbling into flowing water introduced through a dip tube, through holes in a diptube, through a porous diptube, or weeping through pores in a top or side wall of the entry, or introduced through the sidewall of the inlet tubing.
The gas introducing means in one embodiment may for example comprise (i) an upper inlet portion with an annular gas introduction passage including a gas-permeable wall bounding a gas flow passage of the upper inlet portion, and through which the silane-removal-enhancing gas may be flowed, (ii) a lower inlet portion including an annular overflow liquid reservoir with an inner wall having an inner wall surface bounding a gas flow passage through the lower inlet portion of the inlet structure, and producing on overflow a falling film of liquid on the inner wall surface and (iii) a gas inlet tube extending into the gas flow passage and terminating at a lower end in one of the upper inlet and lower inlet portions of the gas introducing means; wherein said gas introducing means is constructed and arranged to introduce silane-containing gas from a source thereof to the scrubbing apparatus.
In another specific embodiment, the gas for enhancing removal of the silane component is introduced to a flow passage of an inlet receiving the silane-containing gas stream, wherein the gas is introduced to the silane-containing gas stream at a central part as well as an outer peripheral part of the gas stream containing silane, so that the silane-abating gas, e.g., clean dry air or other oxygen-containing gas, is intimately mixed with the gas stream to effect cold combustion removal of the silane component, by oxidation thereof.
The aforementioned arrangement may be utilized in combination with a wetted-wall inlet structure of a type as more fully described hereinafter.
A still further aspect of the invention concerns a scrubbing system for treatment of an effluent gas including acid gas components and water-scrubbable components other than acid gas components. Such scrubbing system comprises:
a first scrubber unit for scrubbing the effluent gas with an aqueous scrubbing liquid to remove the acid gas components thereof, said first scrubber unit being constructed and arranged for co-current flow contacting of the aqueous scrubbing liquid and effluent gas with one another to yield effluent gas reduced in acid gas components, as well as water-scrubbable components other than acid gases, as well as water-reactive gases;
a second scrubber unit for scrubbing the effluent gas with a second aqueous scrubbing liquid to remove residual acid gas components and water-scrubbable components other than acid gas components thereof as well as water-reactive components, said second scrubber unit being constructed and arranged for counter-current flow contacting of the second aqueous scrubbing liquid and effluent gas with one another to yield effluent gas reduced in acid gas components and water-scrubbable components other than acid gas components as well as water-reactive components; and
means for flowing the effluent gas reduced in acid gas components from the first scrubber unit to the second scrubber unit.
In the above-described scrubbing system, the acid gas components and water-soluble/water-reactive components will be reduced in the first scrubber unit to concentrations approaching those corresponding to the respective equilibrium values of the acid gas components and water-soluble/water-reactive components in the aqueous scrubbing liquid.
A yet further aspect of the invention involves a gas/liquid contacting article, for removable installation in a scrubber vessel having means for introducing a gas and liquid to an interior volume of the scrubber vessel for gas/liquid contacting therein, such packing medium assembly comprising a fluid-permeable containment structure, e.g., a foraminous bag and a mass of packing elements contained in such fluid-permeable containment structure.
As used herein, the term xe2x80x9cforaminousxe2x80x9d means perforate or otherwise including openings, interstices, vias, or other passages or open space which affords the structure the ability to accommodate fluid flow therethrough. The open space in the foraminous structure may be varied depending on the size of the packing elements to be contained therein.
The invention in a further aspect relates to a scrubbing process for the abatement of a gas component in a gas stream containing same, said scrubbing process comprising introducing the gas stream and a scrubbing liquid to a gas/liquid contacting chamber and effecting gas/liquid contacting therein, wherein said process additionally at least one of the steps of:
(a) introducing a chemical reagent for contact with the gas component to remove same from the gas stream in said gas/liquid contacting;
(b) introducing to the gas stream prior to entry thereof into the contacting chamber, a gas to enhance removal of silane from the gas stream when present therein;
(c) flowing the effluent gas from the contacting chamber to a second gas/liquid contacting chamber and introducing to said second contacting chamber a second scrubbing liquid for gas/liquid contacting therein, wherein the first gas/liquid contacting in the first chamber comprises cocurrent flow of the gas stream and scrubbing liquid and wherein the second gas/liquid contacting in the second contacting chamber comprises countercurrent flow of the gas stream and the second scrubbing liquid through the second contacting chamber;
(d) introducing an antifoam agent to scrubbing liquid for said gas/liquid contacting, to suppress foam production in the contacting chamber, optionally in combination with inducing a back pressure on the scrubbing liquid to supress form production in the contacting chamber,
(e) suppressing deposition of calcium carbonate from scrubbing liquid containing calcium, including a step selected from the group consisting of:
(1) imposing a magnetic field on scrubbing liquid prior to use thereof in the contacting chamber;
(2) adjusting the pH of the scrubbing liquid to maintain pH thereof below 8.5;
(3) flowing the scrubbing liquid through a lime-soda ash bed prior to use of the scrubbing liquid in the contacting chamber; and
(4) precipitating the calcium content of the scrubbing liquid prior to use of the scrubbing liquid in the contacting chamber; and
(f) suppressing solids formation in a passage of the scrubbing system, including a step selected from the group consisting of flowing a purge gas through the passage to suppress solids formation therein, and heating the passage and/or gas flowed therethrough to suppress solids formation therein.
In another aspect, the invention relates to a process for abatement of fluorocompound from an effluent stream containing same, including contacting the gas stream with an aqueous medium in the presence of a reducing agent, such as sodium thiosulfate, ammonium hydroxide, potassium iodide, or any other suitable reducing agent.
In a further aspect, the invention relates to an apparatus for abatement of fluorocompound in an effluent stream containing same, including a water scrubber unit joined in flow relationship with the stream of fluorocompound-containing effluent and arranged for discharge of a fluorocompound-depleted effluent stream, with means for injecting a reducing agent such as sodium thiosulfate, ammonium hydroxide, potassium iodide, or the like into the water scrubber unit to abate the fluorocompound therein and provide an enhanced extent of removal of the fluorocompound, relative to a corresponding system lacking such reducing agent injection.
A still further aspect of the invention relates to a semiconductor manufacturing facility, comprising:
a semiconductor manufacturing process unit producing an effluent gas stream containing a fluorocompound; and
an apparatus for abating fluorocompound in the effluent gas stream, comprising:
a water scrubber unit for gas/liquid contacting;
means for introducing the fluorocompound-containing effluent gas stream to the water scrubber unit;
means for discharging a fluorocompound-reduced effluent gas stream from the water scrubber unit; and
a source of a reducing agent, operatively coupled with the water scrubbing unit and arranged for introducing reducing agent to the water scrubber unit during operation thereof.
The semiconductor manufacturing process unit in such facility may be of any suitable type, as for example a plasma reaction chamber, chemical vapor deposition chamber, vaporizer, epitaxial growth chamber, or etching tool.
Another aspect of the invention relates to an effluent abatement scrubbing system comprising a water scrubber for scrubbing of an effluent gas, said system being constructed and arranged for performing at least one of the functions selected from the group consisting of:
(1) water scrubbing of effluent gas with addition or injection of chemical reducing reagents;
(2) water scrubbing of effluent gas containing silane, wherein clean dry air is introduced to the effluent gas or scrubbing liquid;
(3) utilizing a two-stage scrubbing system including an equilibrium scrubbing column and a polishing mass transfer column, to decrease required make-up water for scrubbing while simultaneously maintaining or increasing scrubbing efficiency relative to a single-stage scrubbing unit;
(4) adding clean dry air to effluent gas discharged from the equilibrium scrubbing column of (3), prior to its introduction to the polishing mass transfer scrubbing column, to abate silane when present with ammonia in the effluent gas stream;
(5) utilizing in a two-stage scrubbing system of (3) a foraminous containment structure containing bed packing as an insert in the polishing mass transfer column;
(6) contacting effluent gas in the scrubbing system with OF2 reducing agents;
(7) controlling foaming in the scrubbing system by chemical antifoam agents and/or orifice restriction of flow of scrubbing liquid;
(8) preventing CaCO3 buildup in the scrubbing system by one or more of the following:
(a) magnetization of make-up water used for scrubbing;
(b) control of the pH of the make-up water;
(c) soda ash-lime softening of the make-up water; and
(d) precipitation or flocculation treatment of the make-up water;
(9) suppressing clogging of a photohelic port including a photohelic sensing line in the scrubbing system, by passing a stream of purge gas through the photohelic sensing line, wherein the photohelic sensing line may optionally be heated; and
(10) heating an inlet structure used in the scrubbing system to introduce effluent gas to a scrubbing zone.
Still other aspects of the invention relate to inlet structures for a gas scrubbing system, removal means and methods for specific gas components to be abated from gas streams containing same, and specific scrubbing system features, techniques, subsystems and approaches.
Other aspects, features and embodiments of the invention therefore are more fully shown hereinafter, and will be more fully apparent from the ensuing disclosure and appended claims.